Repurchase Agreements - Maturities (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 691,666,000	$ 336,998,000
Repurchase agreements	651,246,345	318,557,054
Weighted Average Borrowing Rate	0.35%	0.39%
Between 2 and 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	523,455,000	326,348,000
Repurchase agreements	493,131,000	308,402,000
Weighted Average Borrowing Rate	0.35%	0.39%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	146,233,000	10,650,000
Repurchase agreements	137,677,000	10,155,000
Weighted Average Borrowing Rate	0.35%	0.37%
Greater Than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	21,978,000
Repurchase agreements	$ 20,438,000
Weighted Average Borrowing Rate	0.36%